Due to the liquid shrinkage and solid shrinkage of liquid metal during solidification, the holes formed at the last solidification part of the casting can not be supplemented by external liquid metal in time, so shrinkage cavity and shrinkage porosity casting defects are formed. Among them, those with concentrated distribution and large volume are called shrinkage cavities, and those with small and scattered distribution are called shrinkage porosity. Shrinkage cavity casting defects of castings mainly include internal and external shrinkage cavity, shrinkage porosity and shrinkage depression, as shown in the figure.
There are two main ideas to prevent casting defects from shrinkage porosity and shrinkage cavity. One is to control the solidification mode of castings, and the other is to adopt more reasonable. Control the solidification mode of castings, that is, adopt various measures to make the castings solidify in sequence according to the distance between the parts and the riser, or gradually ensure that the temperature difference between the structures of each part is small, so that all parts solidify at the same time. In order to realize simultaneous solidification, the common concrete measures are: setting the ingate at the thin wall of the casting; Add overflow trough or overflow riser at the thin-walled parts to slow down the cooling speed of these parts; Chill iron or high heat storage molding sand shall be set at the bulge and wall junction where shrinkage porosity and shrinkage cavity are easy to occur.
By improving the casting process to prevent shrinkage porosity and shrinkage cavity, it is necessary to adjust the pouring temperature and pouring speed, and use riser and cold iron when necessary. Generally, the top pouring system uses high-temperature slow pouring, while the bottom pouring system uses low-temperature slow pouring combined with supplementary pouring riser to strengthen sequential solidification. Riser and cold iron are also the most effective process means to prevent shrinkage cavity casting defects. Generally, the riser is set at the thick part or hot joint of the casting, and its size shall ensure that enough liquid metal can be provided for the position of shrinkage hole. At the same time, the feeding channel between the riser and the feeding part shall be kept unblocked during the whole feeding process. The cold iron is usually used with the riser to form an artificial feeding channel and end area, so that the riser can obtain a longer effective feeding distance.